The present invention relates to a method for producing a micro-porous diaphragm or membrane for filtration plants, whereby finely distributed, insoluble particles are mixed and aligned in a plastics material or previously produced or primary plastics material product and are dissolved out on the final position. Such diaphragms or membranes are suitable for ultra filtration of aqueous media, for reverse osmosis and dialysis.
There is generally understood by the term "ultra filtration" the separation of colloidal particles with moderate excess pressure, whilst by reverse osmosis there is understood the separation of substantially smaller, truly dissolved particles at high pressure from the solvent or increasing the concentration of dissolved particles in a solvent.
The membrane forms the core member of a filtration plant. Its properties also determine whether the plant is sufficiently efficient and competitive. A perfect membrane should have the following properties:
(1) it should have, with a moderate charge pressure, a possibly high filter performance PA0 (2) it should have an as uniform as possible width of pores with a sharp upward and downward definition--excessively narrow pores impair the filter performance whilst excessively wide pores lead to the escape of undesired particles; PA0 (3) the pores should be as smooth as possible (capillary structure) and lead towards the filter surface with a sharp-edge (such membranes have a low pressure loss and are less prone to blockages); PA0 (4) the membrane should be constant over a wide pH range. The membrane should not be subjected to microbial reduction and should be inert to an as large as possible number of chemicals and be insensitive to increased working temperature, increased pressures and vibration; PA0 (5) the membrane should be stored as dry as possible to prevent the filter performance from being reduced; PA0 (6) since water absorption, polarity, and wetting angle of the membrane polymer affect the separation selectivity and the passage resistance, these should be freely selectable for the intended purpose; PA0 (7) the membrane is to be producible in accordance with a method which is also perfectly controllable in the production scale and permits a close classification with a low waste quota. PA0 the type and concentration of the polymer, PA0 the type and concentration of the expanding agents, PA0 the type and concentration of the solvents, PA0 the type and concentration of the precipitating agent, PA0 the degree of maturity of the solution, PA0 the layer thickness, temperature, air moisture, air velocity and tempering or curing temperature.
(Definition: cm.sup.3 filtrate/cm.sup.2 /bar/hr., temperature);
Efficient membranes are known which generally comprise an asymmetrically built-up porous layer of plastics material, such as cellulose acetate, polyamide, polyacrylonitrile, etc. These are produced by pouring plastics material solutions of complex composition to form a layer, evaporating or precipitating the smooth close-pored "active" surface thus obtained and the layer located directly beneath formed by coagulation with suitable media into a relatively coarse-pored support layer. Such membranes are at present at a high state of development and their possible disadvantages involving pore width, pore width distribution and thickness of the active layer are subjected to a plurality of influences certain only of which are now listed, namely:
Moreover the number of polymers which are suitable for producing asymmetrical membranes is limited. The manufacturer is therefore not necessarily in a position to provide a membrane substance which on account, for example, of the required chemical resistance, wetting ability and mechanical properties is best suited for the intended purpose.
Filter layers are known, which are produced by limited sintering (firing) of metal ceramics, carbon or polymeric material powders.
Frequently the side of a membrane facing the filtrate is also provided with a fine-pored sinter or suspension layer (so called composite or laminate membranes). Such membranes do not optimally conform to the aforesaid requirements either because the flow line of an imaginary liquid particle is considerably reticulated by the separating layer thus causing a high flow resistance.
British patent specification No. 1,331,775 discloses a method of producing porous polytetrafluorethylene strips in which metal or glass fibres are admixed with a plastics material and moulded into ingots by pressure and wherein the fibres are aligned radially at right angles to the direction of pressure. A thin sheet is obtained by a peeling action and the fibres in the sheet, aligned substantially at right angles to the sheet surface, are then washed out. The peeling phase in particular is difficult to carry out and too costly for industrial production of the initially mentioned micro-porous membrane.
An object of the present invention is to produce a microporous membrane which fulfils the aforesaid conditions and does not contain the disadvantages described of the prior art membranes.